[ecash-script] Add option to return pushdata to existing consumeNextPush function

Summary: Add optional param returnPushdata which will cause consumeNextPush to return {push, pushdata} instead of push

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14011